The description of prior art will be primarily based on the list of references presented at the end of this section.
Lithium-ion and lithium (Li) metal batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium metal has the highest capacity compared to any other metal or metal-intercalated compound as an anode material. Hence, Li metal batteries have a significantly higher energy density and power density than lithium ion batteries. However, cycling stability and safety concerns remain the primary factors preventing the wide-scale commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. Specific cyclic stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway.
Many attempts have been made to address the dendrite-related issues, as summarized below:
Foster [Ref. 1] proposed a multilayer separator that included a porous membrane and an electro-active polymeric material contained within the separator materials. The polymer is capable of reacting with any lithium dendrite that might penetrate the separator, thus preventing the growth of dendrites from the anode to cathode that otherwise would cause internal shorting.
In a technically similar fashion, Shen, et al. [Ref. 2], used a non-reactive first porous separator (e.g., porous polypropylene) adjacent to the lithium anode and a second fluoro-polymer separator between the cathode and the first separator. The second separator (e.g., polytetrafluoro ethylene) is reactive with lithium. As the tip of a lithium dendrite comes into contact with the second separator, an exothermic reaction occurs locally between the lithium dendrite and the fluoro-polymer separator, resulting in the prevention of the dendrite propagation to the cathode.
Goebel, et al. [Ref. 3], proposed a “getter” electrode positioned between the anode and the cathode and was separated from the cathode and anode by fiberglass paper separators. The getter layer, composed of carbon or graphite material disposed on surfaces of these separators, serves as a low-capacity cathode that quickly discharges any Li dendrite that comes in contact with the getter layer.
Fauteux, et al. [Ref. 4], applied to a metal anode a surface layer (e.g., polynuclear aromatic and polyethylene oxide) that enables transfer of metal ions from the metal anode to the electrolyte and back. The surface layer is also electronically conductive so that the ions will be uniformly attracted back onto the metal anode during electrodeposition.
Alamgir, et al. [Ref. 5], used ferrocenes to prevent chemical overcharge and dendrite formation in a solid polymer electrolyte-based rechargeable battery.
Kawakami, et al. [Ref. 6], observed that internal shorting could be prevented by using a multi-layered metal oxide film as a separator with small apertures through which lithium ions can pass and the growth of dendrites can be inhibited. Kawakami, et al. [Ref. 7], further suggested a first thin film coating on the anode and a second thin film coating on the cathode, with both coatings permeable to lithium ions, could be effective in preventing dendrite formation. The first film can contain a large ring compound, an aromatic hydrocarbon, a fluoro-polymer, a glassy metal oxide, a cross-linked polymer, or a conductive powder dispersion. However, the dendrite-preventing mechanisms of these films were not clearly explained. Kawakami, et al. [Ref. 8], also found that some size mismatch between the anode and the cathode (with the anode being larger) seems to be effective in preventing dendrite formation.
Zhang [Ref. 9] disclosed a separator that is composed of a ceramic composite layer (to block dendrite growth) and a polymer micro-porous layer (to block ionic flow between the anode and cathode in the event of a thermal runaway).
Skotheim [Ref. 10] provided a Li metal anode that was stabilized against dendrite formation by the use of a vacuum-evaporated thin film of a Li ion-conducting polymer interposed between the Li metal anode and the electrolyte. Ying, et al. [Ref. 11], proposed a separator that comprises a microporous pseudo-boehmite layer and a polymer-based protective coating layer. It was speculated that this separator had a small pore structure (10 μm or less) and sufficient mechanical strength to prevent the Li dendrite from contacting the cathode and causing internal shorting. Skotheim, et al. [Ref. 12], proposed a multilayer anode structure consisting of a Li metal-based first layer, a second layer of a temporary protective metal (e.g., Cu, Mg, and Al), and a third layer that is composed of at least one layer (typically 2 or more layers) of a single ion-conducting glass, such as lithium silicate and lithium phosphate, or polymer. It is clear that such an anode active material, consisting of at least 3 or 4 layers, is too complex and too costly to make.
Protective coatings for Li anodes, such as glassy surface layers of LiI—Li3PO4—P2S5, may be obtained from plasma assisted deposition [Ref. 17]. Complex, multi-layer protective coatings were also proposed by Visco, et al. [Ref. 18].
Organic additives that were used to stabilize the lithium anode active surface include (a) an organosilicon backbone with pyridinium groups bound to the backbone [Ref. 13], (b) halogenated organic metal salts [Ref. 14], and (c) dioxolane [Ref. 15]. Nimon, et al. [Ref. 16], developed methods and reagents for enhancing the cycling efficiency of lithium polymer batteries. The methods entailed forming a protective layer (e.g., LiAlCl4.3SO4 and Al2S3) on the lithium metal anode surface through a reaction of electrolyte species with lithium metal.
Despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes are too laborious or difficult. Clearly, an urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries and other rechargeable batteries.